Runs a piece of middleware for each already received context object every time a context object is received. This can be used to install plugins inside conversations.

For instance, if three context objects arrive, this is what happens:

1. the first update is received
2. the middleware runs for the first update
3. the second update is received
4. the middleware runs for the first update
5. the middleware runs for the second update
6. the third update is received
7. the middleware runs for the first update
8. the middleware runs for the second update
9. the middleware runs for the third update

Note that the middleware is run with first update thrice.